Such a statement is incomprehensible to those in the political class. They view Bush and Clinton as experienced hands who know how the game of politics and government is supposed to be played. But that’s precisely what most voters hate. They are fed up with the status quo. If the definition of insanity is doing the same thing over and over again while expecting a different outcome, many voters see a Bush-Clinton match as simply insane.
To be clear, a Trump-Sanders match-up is even less likely than Bush-Clinton. But the reality of Election 2016 is that many voters would prefer the extremists to the insiders. Moving beyond Trump and Sanders, a majority of voters in each party would probably prefer a choice between Ted Cruz and Elizabeth Warren rather than Clinton vs. Bush.
The hatred of the status quo is stronger among Republicans than Democrats. That makes it more difficult for Bush to win the nomination than Clinton. Another challenge for the choice of the GOP establishment is the fact that Trump is dominating the campaign chatter.